It is a commonly used technique to detect a target by using active light source illumination. FIG. 1 shows a schematic diagram of the process of the existing target detection method. As shown in FIG. 1, several main modules including a light source, an image sensor, a synchronization mechanism and an image processor are generally involved. Features of a target object, such as its position, shape, speed, color, and texture, may be detected in real time by such modules in an operation principle as follows: the light source emits light to illuminate the target object; and the image sensor obtains the image of the entire scene including the target objet and background objects behind the target object, under the control of the synchronization mechanism. The target object may be an ordinary object having a diffuse surface, or a prefabricated special object, that has a surface producing a directional reflection of light so that the light emitted from the light source may be reflected to the image sensor. Since the target object is closer to the light source than the background, or it has the special reflective surface, the brightness of the target object is much higher than that of the background in the image. The image processor receives and divides the image, and a portion of the image with brightness higher than a certain threshold is regarded as the target object, and a portion of the image with brightness lower than the threshold is regarded as the background. As a result, subsequent operations, such as extracting position information, are performed according to the division of the target area.
The currently available detection method as above has the disadvantage that the extraction of the target is susceptible to interference by an ambient light. If the irradiation of the ambient light causes the brightness of a certain region in the background to be higher than that of the target object, the above method will be ineffective.
In the prior art, the general solution for overcoming the interference of the ambient light is to modulate the light source to a specific wavelength λ (which is generally out of the range of visible wavelengths in order not to affect observation by human eyes), increase the output power, and arrange a band pass filter corresponding to the wavelength λ between the image sensor and the target object, so that only the light of the wavelength λ is allowed to pass. This method can suppress the effects of the ambient light to some extent, and improve the contrast between the target object and the background. However, since the ambient light (such as the sunlight and the light of an incandescent lamp) generally contains full wavelengths, and its light intensity at the wavelength λ may also be much larger than that of the light emitted from the light source, the method based on the selected specific wavelength may also be failed.
At present, a new method has been proposed to solve above problems. The method uses an ordinary Complementary Metal Oxide Semiconductor (CMOS) image sensor, and obtains continuously two frames of images I1 and I2 from the same scene (assuming that the positions of the target object and the background object are substantially not changed throughout the entire process), here, the image I1 is obtained with the light from the light source, and the image I2 is obtained without the light from the light source. Since the target object is closer to the light source or has a special reflective surface, the brightness of the target object in image I1 is much higher than that in the image I2; on the contrary, since the background object is farther from the light source and has no special reflective surface, the brightness of the background object in the image I1 is not significantly different from that in the image I2. The images I1 and I2 are received by the image processor and subjected to a subtracting operation, resulting in an image I in which the brightness of the target object is much larger than that of the background object. The image I is divided, and a portion of the image I with brightness higher than a certain threshold is regarded as the target object, and a portion of the image I with brightness lower than the threshold is regarded as the background. As a result, subsequent operations, such as extracting positional information, are performed according to the division of the target area.
Such a new method solves the problem of the ambient light radiation, but is disadvantageous in the real-time performance of the entire system and the utilization efficiency of the light source energy. This is related to the operating mode of the CMOS image sensor.
Reference is now made to FIG. 2, which shows a diagram of the operating mode of the existing CMOS image sensor. For most CMOS image sensors, different exposure times are provided for each pixel row. The abscissa in FIG. 2 represents a time axis. As shown in FIG. 2, after a while from starting the exposure of the first pixel row L1, the exposure of the second pixel row L2 starts, and likewise, the exposures of the following pixel rows are started sequentially. The ends of the exposures of the pixel rows are also sequential.
In such an exposure manner, in order that various pixel rows receive the same amount of light emitted from the light source, only two flash time solutions of the light source, namely solutions denoted by Flash 1 and Flash 2 as shown in FIG. 2, can be selected. In the solution Flash 1, the flash is started at the time when the exposure of the last pixel row starts and finished at the time when the exposure of the first pixel row ends. In the solution Flash 2, the flash is started at the time when the exposure of the first pixel row starts and finished at the time when the exposure of the last pixel row ends. Both the two flash solutions have not reached the optimization of time and efficiency. In the solution Flash 1, the flash time of the light source is less than the exposure time of the CMOS image sensor, thus the exposure time is not fully utilized to increase the brightness of the target object. In the solution Flash 2, the flash time of the light source is more than the exposure time of the CMOS image sensor, which causes an insufficient use of the light source energy.
In addition, if the exposure time is increased, the proportion of the interval between the flash time and the exposure time to the total exposure time is gradually reduced. To the upmost degree, the flash time is approximately equal to the exposure time, which is however not advisable, because this solution is used in the real-time target detection system in which the target moves at a certain speed, and the theory of the above method is based on the assumption that the position of the target is almost not changed in the two frames of continuously photographed images. Increasing the exposure time would make this assumption invalid, thus this solution is ineffective.
Therefore, the above problem can be solved if it is ensured that all the pixel rows are synchronously exposed. Reference is now made to FIG. 3, which is a diagram of the operating mode of the CMOS image sensor in the case that all the pixel rows are synchronously exposed. If the flash time is configured to be synchronous with the exposure time, each pixel row may receive the same amount of light emitted from the light source.
However, it is difficult for the current CMOS image sensors to achieve the exposure synchronization among all the pixel rows. The description below is made in detail in conjunction with the circuitry of a pixel unit in the CMOS sensor. Reference is now made to FIG. 4, which is a diagram of the circuitry of the pixel unit of the existing CMOS sensor. The CMOS sensor is an array constituted by a number of such pixel units. As shown in FIG. 4, the pixel unit includes a photocell B, a reset transistor R, a charge overflow transistor T, a source follower FD and a row strobe transistor X. Furthermore, pixel units in each pixel column are connected commonly to a pair of signal outputting transistors, i.e., a first signal outputting transistor SH1 and a second signal outputting transistor SH2. The reset transistor R, the charge overflow transistor T and the photocell B are successively connected in series between an active power and the ground. One end of the source follower FD is connected to the active power, the other end of the source follower FD is connected to the row strobe transistor X, and the gate of the source follower FD is connected to a node between the reset transistor R and the charge overflow transistor T. The other end of the charge overflow transistor T is respectively connected to an end of the first signal outputting transistor SH1 and an end of the second signal outputting transistor SH2 of the pixel column containing the pixel unit.
Reference is now made to FIG. 5, which is a control timing diagram of the pixel unit in the existing CMOS image sensor. As shown in FIG. 5, before the starting of the exposure of pixels in each pixel row, the reset transistor R and the charge overflow transistor T are turned on, and charges in the photocell B and the source followers FD are emptied. After that, the reset transistor R and the charge overflow transistor T are turned off and the exposure starts. The photocell B starts to accumulate charges. Before the exposure time T1 ends, the row strobe transistor X and the first signal outputting transistor SH1 are turned on to sample the reference level of the source follower FD, and then turned off immediately. At the end of the exposure, the charge overflow transistor T is turned on to transfer the charges in the photocell B to the source follower FD, and then the charge overflow transistor T is turned off immediately. The row strobe transistor X and the second signal outputting transistor SH2 are turned on to sample the signal level of the source follower FD. A digital signal is obtained from the comparison between the second signal outputting transistor SH2 and the first signal outputting transistor SH1. It is noted that this is just the exposure control timing of the circuit contained in a certain pixel unit of the CMOS image sensor. In the entire CMOS image sensor, since the CMOS image sensor needs to output data in series, the exposure control timing of pixel units in the same pixel row is synchronous, while the exposure control timing of pixel units in different rows is successively executed in a time order, so that the time for outputting data by each pixel row is not conflicted with another pixel row.
It can be known in conjunction with FIG. 2 to FIG. 5 that, before the end of the exposure time, the row strobe transistor X initiates the sampling of the level, that is, starts to output the data of the pixel row. Thus, in the prior art, data must be immediately output after the exposure in this timing control method. Since the CMOS image sensor is required to output the data in series, only if the exposures of the various pixel rows is started sequentially, the data can be outputted sequentially when the exposure of each pixel row is finished. When the exposure time of all pixel rows is synchronized, the time for outputting data by all the pixel rows is also synchronized, which will inevitably cause that the image data cannot be normally outputted.